In the case of such two-coil actuators it is known to detect the position of the armature arranged and able to move between the coils, without needing an additional sensor. For this purpose the two coils are connected in series and a voltage jump is applied to the arrangement so that the two coils form a voltage divider in accordance with the impedance coil principle known from measurement technology. The position of the armature can be determined from the difference between the two voltages produced. For example, when the armature is in the first coil the voltage at the first coil at the beginning of the jump response will be higher than the voltage in the second coil; from the ratio of the voltages relative to the magnetic rise, a position-dependent displacement signal can be generated in a simple manner.
For example, from DE 10 2005018012 A1 by the present applicant an electromagnetic actuator and a method for controlling the actuator are known, the actuator consisting of an armature and two coils. In DE 10 2005108012 A1 it is proposed to measure the voltage variation at the two coils during a sudden increase of energization, so that from these measurement data a third voltage variation is calculated in a differential imager, from which a logic unit determines the position of the armature arranged and able to move between the coils, without using an additional sensor.
However, the displacement signal generated in this manner depends markedly on the size and shape of the voltage jump which, owing to conductor losses or fluctuations of the on-board electric voltage, is neither constant nor always has the same value in practice. Furthermore, with an increasing value of the time when the measurements are made after the voltage jump, the measured value is influenced adversely by temperature effects due to the ohmic fraction and by magnetic saturation effects.
From DE 19748647 C2 an electromagnetic drive system with integrated displacement signal production is known, which comprises an electric motor and consists of a first part-system having at least one permanent magnet and a second part-system with a coil arrangement comprising at least two identical part-coils arranged one behind the other. In this known system the respective part-systems form the stator or the movable armature; in addition a control circuit is provided, which serves to produce a pulse-width-modulated control voltage with constant pulse frequency and a constant switching voltage in the coil system. The known system includes an evaluation circuit which, by respective separate differentiation of the voltage variations across the part-coils and subsequent subtraction or division of the then produced new differentiated part-voltages at a respectively constant time-point after the flank inversion in the pulse-width-modulated control signal, derives a voltage value proportional to displacement or angle for the relative position between the first part-system of the motor and the second part-system. Thanks to the concept of the dual use of the drive coils there is no need for separate measurement systems for determining the position of the moving part-system relative to the fixed part-system of the drive system.